1. Field of the Invention
The present invention generally relates to radio communications systems to which Orthogonal Frequency Division Multiplexing (OFDM) is applied in downlink and specifically relates to base station apparatuses, mobile station apparatuses, and methods of transmitting synchronization channels.
2. Description of the Related Art
As a communications scheme to succeed W-CDMA (Wideband Code Division Multiple Access) and HSDPA, Long Term Evolution (LTE) is being studied in a W-CDMA standardization body called 3GPP. Moreover, as radio access schemes, the OFDM is being considered for downlink, while SC-FDMA (Single-Carrier Frequency Division Multiple Access) is being considered for uplink (see Non-patent document 1, for example).
The OFDM, which is a scheme for dividing a frequency band into multiple narrow frequency bands (sub-carriers) and overlaying data onto the respective frequency bands for transmission, densely arranges the sub-carriers on the frequency axis such that one sub-carrier partially overlaps another sub-carrier without their interfering with each other, making it possible to achieve high-speed transmission and to improve frequency utilization efficiency.
The SC-FDMA is a transmission scheme which divides a frequency bandwidth and transmits using different frequency bands among multiple terminals to make it possible to reduce interference between the terminals. The SC-FDMA, which features a reduced variation in transmission power, makes it possible to achieve wide coverage as well as low power consumption of the terminals.
In the LTE, the OFDM provides for two types of CPs (Cyclic Prefixes) for reducing the effect of intersymbol interference by a delay wave, namely a Long CP and a Short CP with different lengths. For example, the Long CP is applied in a cell with a large cell radius and at the time of transmitting an MBMS (Multimedia Broadcast Multicast Service) signal, while the Short LP is applied in a cell with a small cell radius. The number of OFDM symbols is 6 when the Long CP is applied and 7 when the short CP is applied.
Now, in a radio communications system using W-CDMA, LTE, etc., a mobile station must generally detect a cell with good radio quality for the own station based on a synchronization (sync) signal, etc., at the time of turning on the power, in a standby status, during communications, or at the time of intermittent reception during communications. The process, which is meant to search for a cell to which a radio link is to be connected, is called a cell search. The cell search method is generally determined based on a time needed for the cell search as well as throughput of the mobile station at the time of conducting the cell search. In other words, the above-described cell search method should be such that the time needed for the cell search is short and the throughput of the mobile station at the time of conducting the cell search is small.
In the W-CDMA, the cell search is conducted using two types of synchronization signals, namely a Primary SCH (P-SCH) and a Secondary SCH (S-SCH). Similarly, conducting the cell search using the two types of the synchronization signals P-SCH and S-SCH is also being considered in the LTE.
For example, a cell search method is being considered such that the P-SCH with one sequence and S-SCH with multiple sequences are transmitted at time intervals of 5 ms (Non-patent document 2). In the above-described method, a downlink receive timing from a cell is specified using the P-SCH, while a receive frame timing is detected and a cell-specific information set such as a cell ID, or cell group ID is specified using the S-SCH transmitted in the same slot. Here, it is generally possible to use a channel estimation value determined from the P-SCH in demodulating and decoding the S-SCH. Then, the cell IDs to be grouped are detected from those cell IDs belonging to the detected cell group ID. For example, the cell ID is calculated based on a signal pattern of a pilot signal. Moreover, the cell ID is calculated based on the demodulation and decoding of the P-SCH and the S-SCH, for example. Alternatively, without grouping the cell IDs, the cell ID may be included as an information element of the S-SCH. In this case, the mobile station can detect the cell ID at the time of demodulating and decoding the S-SCH.
However, in an inter-station synchronization method in which signals from multiple cells are being synchronized, when the above-described cell search method is applied, a problem occurs such that the S-SCHs transmitted from multiple cells in different sequences are demodulated and decoded based on the channel estimation value determined from the P-SCHs transmitted from multiple cells in the same sequence. Here, the transmission characteristics also include a time needed for the cell search, for example. For a non-inter-station synchronization system in which signals from multiple cells are not being synchronized, receive timings of the P-SCH sequences transmitted from the multiple cells differ from one cell to another. Thus, such a problem as described above does not occur.
In order to prevent a degradation in the S-SCH characteristics in the inter-station synchronization system as described above, a cell search method is being considered such that the number of the P-SCH sequences is increased from 1 to a number no less than 2 (for example, 3 or 7) (see Non-patent document 3). Alternatively, there is a method of transmitting the P-SCH in transmission intervals which differ on a per cell basis in order to prevent the S-SCH characteristics degradation in the inter-station synchronization system as described above. In the above-described method, the P-SCHs having different timings of receiving from the multiple cells may be used in the demodulating and decoding of the S-SCH. Thus, it is made possible to prevent the S-SCH characteristic degradation as described above.
Now, from a point of view of cell design, it is deemed that the larger the number of sequences of the P-SCH in Non-patent document 3 and the types of transmission intervals of the P-SCH in Non-patent document 4, the better they are. This is because, the smaller the number of sequences of the P-SCH or the types of transmission intervals, the higher the probability of the P-SCH sequences in neighboring cells becoming the same, or the higher the probability of the P-SCH transmission intervals becoming the same, so that the probability of occurrence of the S=SCH characteristic degradation in the inter-station synchronization system becomes higher.
Moreover, there is a tradeoff relationship between the time needed for conducting the cell search as described above, or the transmission characteristics of the cell search, and the throughput of the mobile station when the cell search is being conducted. Thus, it is desirable to be able to select whether the transmission characteristics of the cell search are to be emphasized or the throughput of the mobile station when the cell search is being conducted is to be emphasized.
Non-Patent Document 1:    3GPP TR 25.814 (V7.0.0), “Physical Layer Aspects for Evolved UTRA,” June 2006;
Non-Patent Document 2:    R1-062990, Outcome of cell search drafting session;
Non-Patent Document 3:    R1-062636, Cell Search Performance in Tightly Synchronized Network for E-UTRA;
Non-Patent Document 4:    R1-070428, Further analysis of initial cell search for Approach 1 and 2—single cell scenario;
Non-Patent Document 5:    3GPP TS 36.211 V1.0.0 (2007-03);
Non-Patent Document 6:    3GPP R1-060042 SCH Structure and Cell Search Method in E-UTRA Downlink;
Non-Patent Document 7:    3GPP R1-071584 Secondary Synchronization Signal Design;
Non-Patent Document 8:    3GPP R1-071794;
Non-Patent Document 9:    Chu, “Polyphase codes with good periodic correlation properties,” IEEE Trans. Inform. Theory, vol. II-18, pp. 531-532, July 1972;
Non-Patent Document 10:    R. L. Frank and S. A. Zadoff, “Phase shift pulse codes with good periodic correlation properties,” IRE Trans. Inform. Theory, vol. IT-8, pp. 381-382, 1962;
Non-Patent Document 11:    M. J. E. Golay, “Complementary Series,” IRE Trans. Inform. Theory, vol. 7, pp. 82-87, April 1961;
Non-Patent Document 12:    3GPP, R1-062487 Hierarchical SCH signals suitable for both (FDD and TDD) modes of E-UTRA;
Non-Patent Document 13:    3GPP, R1-070146, S-SCH Sequence Design;
Non-Patent Document 14:    3GPP, R1-072093, Details on SSC Sequence Design;
Non-Patent Document 15:    3GPP, R1-071641, Frequency Hopping/Shifting of Downlink Reference Signal in E-UTRA;
Non-Patent Document 16:    3GPP, R1-072368, Mapping of Short Sequences for S-SCH;
Non-Patent Document 17:    3GPP, R1-072326, S-SCH sequences based on concatenated Golay Hadamard codes;
Non-Patent Document 18:    3GPP, R1-072189, Views on Remaining Issues on SCH Design;
Non-Patent Document 19:    3GPP, R1-072328, Secondary-Synchronization Channel Design;
Non-Patent Document 20:    3GPP, R1-072110, Secondary Synchronisation Codes for LTE cell search; and
Non-Patent Document 21:    3GPP, R1-072661, Scrambling Method for Two S-SCH Short Code